An improvement of fuel efficiency of vehicles has been recently demanded from the viewpoint of global environmental conservation. In addition, improvements in automobile safety have been required in order to protect passengers in the event of a crash. Thus, demands have been placed on both weight saving and strengthening of car bodies, and thinning and strengthening of automobile parts have been recently pursued.
Because a lot of automobile parts are manufactured by press forming of steel sheets, it has been strongly demanded that steel sheets used for automobile parts exhibit excellent press formability as well as high strength. Solid solution strengthening by the addition of Si is a known technique which increases the strength of steel sheets without a marked deterioration in press formability.
However, solid solution strengthening has a problem in that if a large amount, in particular, 0.5 mass % or more, of Si is added to cold rolled steel sheets, Si-containing oxides such as SiO2 (silica) and SiMnO3 (manganese silicate) are formed on the surface of steel sheets during annealing. These Si-containing oxides inhibit the surface of steel sheets from being etched during zinc phosphate treatment (chemical conversion treatment) performed as a base treatment before the steel sheets are electrodeposition coated. Consequently, formation of sound chemical conversion film is inhibited. When such high-strength cold rolled steel sheets having a high Si content are exposed, after being electrodeposition coated, to severe environments such as hot saline water immersion tests or complex cycle corrosion tests in which the steel sheets are wetted and dried repeatedly, the coatings are stripped easily as compared to usual steel sheets. That is, post-coating corrosion resistance deteriorates easily.
Many proposals have been made of methods for improving the chemical conversion properties of Si-containing steel sheets, in particular, steel sheets having a high Si content of not less than 0.5 mass %.
Patent Literature 1 proposes a technique in which Mn/Si ratio is controlled to be not less than 1.2 so as to suppress formation of inactive Si oxides on the surface and to promote generation of active Mn oxides. Thereby, good chemical conversion property is realized even in the case of box annealing of high-Si steel.
Patent Literature 2 proposes a technique in which good chemical conversion property is obtained by attaching 20 to 1500 mg/m2 of iron to the surface of sufficiently clean cold rolled steel sheets, although the mechanism of this effect is not fully understood.
Patent Literature 3 proposes a technique directed to improve chemical conversion properties by controlling the rate of coverage on the steel sheet surface by Si oxides and the size of Si oxide. The technique controls the dew point during continuous annealing to 0° C. to −20° C. and Si oxides on the surface layer are removed with concentrated hydrochloric acid or concentrated sulfuric acid after the continuous annealing.
Patent Literature 4 proposes a technique in which a portion of steel sheet extending 1 μm or more from the surface on each side is removed by pickling to eliminate all the oxides present in the steel. Thereby, excellent chemical conversion properties are obtained.
Patent Literature 5 proposes a method in which Si oxides formed on the surface of steel sheet during annealing are removed by pickling and immediately thereafter the steel sheet is brought into contact with a sulfur compound. As a result, the number of zinc phosphate crystal nuclei is increased. Thereby, the size of zinc phosphate crystals is reduced and the density of zinc phosphate crystals is increased and thus chemical conversion properties are improved.
Before the steel sheets obtained in accordance with these literatures are subjected to chemical conversion treatments, surfaces of the steel sheets are usually pickled with acids to remove oxide layers present on the surfaces of the steel sheets after the continuous annealing. In the case where the pickling is performed continuously, the acid concentration in the pickling solution is decreased with the consumption of acids and consequently the pickling performance decreases. In order to prevent the decrease in the pickling performance of the pickling solution and to ensure a constant pickling performance, it is necessary that the acid concentration in the pickling solution is measured on a regular basis and the supplementary acids are added to the pickling solution.
The following analytical methods have been known as methods for the regular measurement of acid concentration in a pickling solution. For example, measuring the concentration of nitric acid in a mixed acid containing nitric acid and hydrofluoric acid is most commonly performed by first determining the total acid concentration in the pickling solution by a neutralization titration method and thereafter subtracting the hydrofluoric acid concentration from the total acid concentration. As the analytical methods for determining the latter concentration, namely, the hydrofluoric acid concentration, for example, Patent Literatures 6 and 7 describe an absorbance method based on discoloration of iron acetylacetone complex and an analytical method based on an ion electrode method, respectively.